Role of Na+ and Li+ in thiomethylgalactoside transport by the melibiose transport system of Escherichia coli

An exploratory study on the possibilities of microalgal biotechnology to obtain the essential 6Li isotope as fusion fuel

Future energy supply needs to overcome two challenges: environmental impact and dependence on geopolitically unstable countries. A very promising alternative is based on lithium, an element for batteries, and whose isotope 6Li will be essential in nuclear fusion. The objective of this research has been to determine if it is possible to achieve isotopic fractionation of lithium through a process mediated by microalgae. For this purpose, Chlamydomonas reinhardtii was selected and grown in presence of 5 mg/L of lithium. Results revealed that this specie survives at the selected lithium concentration, discriminates isotopes and preferentially capture 6Li (6δ = 10.029 ± 3.307) through a process independent of the cellular growth. Concomitate recovered up 0.206 mg/L of lithium along a process of 21 days. The result of this study lets to affirm that Chlamydomonas reinhardtii might be used to obtain lithium enriched in the lighter isotope.

Complete cysteine-scanning mutagenesis of the Salmonella typhimurium melibiose permease

The melibiose permease of Salmonella typhimurium (MelB_St) catalyzes the stoichiometric symport of galactopyranoside with a cation (H⁺, Li⁺, or Na⁺) and is a prototype for Na⁺-coupled major facilitator superfamily (MFS) transporters presenting from bacteria to mammals. X-ray crystal structures of MelB_St have revealed the molecular recognition mechanism for sugar binding; however, understanding of the cation site and symport mechanism is still vague. To further investigate the transport mechanism and conformational dynamics of MelB_St, we generated a complete single-Cys library containing 476 unique mutants by placing a Cys at each position on a functional Cys-less background. Surprisingly, 105 mutants (22%) exhibit poor transport activities (<15% of Cys-less transport), although the expression levels of most mutants were comparable to that of the control. The affected positions are distributed throughout the protein. Helices I and X and transmembrane residues Asp and Tyr are most affected by cysteine replacement, while helix IX, the cytoplasmic middle-loop, and C-terminal tail are least affected. Single-Cys replacements at the major sugar-binding positions (K18, D19, D124, W128, R149, and W342) or at positions important for cation binding (D55, N58, D59, and T121) abolished the Na⁺-coupled active transport, as expected. We mapped 50 loss-of-function mutants outside of these substrate-binding sites that suffered from defects in protein expression/stability or conformational dynamics. This complete Cys-scanning mutagenesis study indicates that MelB_St is highly susceptible to single-Cys mutations, and this library will be a useful tool for further structural and functional studies to gain insights into the cation-coupled symport mechanism for Na⁺-coupled MFS transporters.

The Upcoming 6Li Isotope Requirements Might Be Supplied by a Microalgal Enrichment Process

Lithium isotopes are essential for nuclear energy, but new enrichment methods are required. In this study, we considered biotechnology as a possibility. We assessed the Li fractionation capabilities of three Chlorophyte strains: Chlamydomonas reinhardtii, Tetraselmis mediterranea, and a freshwater Chlorophyte, Desmodesmus sp. These species were cultured in Li containing media and were analysed just after inoculation and after 3, 12, and 27 days. Li mass was determined using a Inductively Coupled Plasma Mass Spectrometer, and the isotope compositions were measured on a Thermo Element XR Inductively Coupled Plasma Mass Spectrometer. The maximum Li capture was observed at day 27 with C. reinhardtii (31.66 µg/g). Desmodesmus sp. reached the greatest Li fractionation, (δ⁶ = 85.4‰). All strains fractionated preferentially towards ⁶Li. More studies are required to find fitter species and to establish the optimal conditions for Li capture and fractionation. Nevertheless, this is the first step for a microalgal nuclear biotechnology.

X-ray crystallography reveals molecular recognition mechanism for sugar binding in a melibiose transporter MelB

Major facilitator superfamily_2 transporters are widely found from bacteria to mammals. The melibiose transporter MelB, which catalyzes melibiose symport with either Na⁺, Li⁺, or H⁺, is a prototype of the Na⁺-coupled MFS transporters, but its sugar recognition mechanism has been a long-unsolved puzzle. Two high-resolution X-ray crystal structures of a Salmonella typhimurium MelB mutant with a bound ligand, either nitrophenyl-α-d-galactoside or dodecyl-β-d-melibioside, were refined to a resolution of 3.05 or 3.15 Å, respectively. In the substrate-binding site, the interaction of both galactosyl moieties on the two ligands with MelB_St are virturally same, so the sugar specificity determinant pocket can be recognized, and hence the molecular recognition mechanism for sugar binding in MelB has been deciphered. The conserved cation-binding pocket is also proposed, which directly connects to the sugar specificity pocket. These key structural findings have laid a solid foundation for our understanding of the cooperative binding and symport mechanisms in Na⁺-coupled MFS transporters, including eukaryotic transporters such as MFSD2A.

Guan and Hariharan report two crystal structures of melibiose transporter MelB in complex with substrate analogs, nitrophenyl-galactoside, and dodecyl-melibioside. Both structures revealed similar specific site for sugar recognition and resolved the cation-binding pocket, advancing the understanding of MelB and related transporters.

Cooperative binding ensures the obligatory melibiose/Na+ cotransport in MelB

MelB catalyzes the obligatory cotransport of melibiose with Na⁺, Li⁺, or H⁺. Crystal structure determination of the Salmonella typhimurium MelB (MelB_St) has revealed a typical major facilitator superfamily (MFS) fold at a periplasmic open conformation. Cooperative binding of Na⁺ and melibiose has been previously established. To determine why cotranslocation of sugar solute and cation is obligatory, we analyzed each binding in the thermodynamic cycle using three independent methods, including the determination of melting temperature by circular dichroism spectroscopy, heat capacity change (ΔC_p), and regulatory phosphotransferase EIIA^Glc binding with isothermal titration calorimetry (ITC). We found that MelB_St thermostability is increased by either substrate (Na⁺ or melibiose) and observed a cooperative effect of both substrates. ITC measurements showed that either binary formation yields a positive sign in the ΔC_p, suggesting MelB_St hydration and a likely widening of the periplasmic cavity. Conversely, formation of a ternary complex yields negative values in ΔCp, suggesting MelB_St dehydration and cavity closure. Lastly, we observed that EIIA^Glc, which has been suggested to trap MelB_St at an outward-open state, readily binds to the MelB_St apo state at an affinity similar to MelB_St/Na⁺. However, it has a suboptimal binding to the ternary state, implying that MelB_St in the ternary complex may be conformationally distant from the EIIA^Glc-preferred outward-facing conformation. Our results consistently support the notion that binding of one substrate (Na⁺ or melibiose) favors MelB_St at open states, whereas the cooperative binding of both substrates triggers the alternating-access process, thus suggesting this conformational regulation could ensure the obligatory cotransport.

Thermodynamic cooperativity of cosubstrate binding and cation selectivity of Salmonella typhimurium MelB

The melibiose symporter MelB couples melibiose transport to that of cations such as Na⁺. Hariharan and Guan show that the binding of Na⁺ and melibiose is thermodynamically cooperative and that Na⁺ coupling is based on ion concentrations and competitive binding, but not ion selectivity.

The Na⁺-coupled melibiose symporter MelB, which can also be coupled to H⁺ or Li⁺ transport, is a prototype for the glycoside-pentoside-hexuronide:cation symporter family. Although the 3-D x-ray crystal structure of Salmonella typhimurium MelB (MelB_St) has been determined, the symport mechanisms for the obligatory coupled transport are not well understood. Here, we apply isothermal titration calorimetry to determine the energetics of Na⁺ and melibiose binding to MelB_St, as well as protonation of this transporter. Studies of the thermodynamic cycle for the formation of the Na⁺–MelB_St–melibiose ternary complex at pH 7.45 reveal that the binding of Na⁺ and melibiose is cooperative. The binding affinity for one substrate (Na⁺ or melibiose) is increased by the presence of the other by about eightfold. The coupling free energies (ΔΔG) of either substrate binding are ∼5 kJ/mol, and binding of both substrates releases a free energy of ∼35 kJ/mol. Measurements of the Na⁺-binding enthalpy at three different pH values, including the pK_a value of MelB, indicate that the binding of one Na⁺ displaces one H⁺ per MelB_St molecule. In addition, the absolute dissociation constants for Na⁺ and H⁺, determined by competitive binding, show that MelB_St is selective for H⁺ over Na⁺ by ∼1,000-fold at a pK_a of 6.25. Thus, the Na⁺ coupling in MelB_St is based not on ion selectivity but on ion concentrations and competitive binding because of a much higher Na⁺ concentration under physiological conditions. Such a selectivity feature seems to be common for membrane transport proteins that can bind both H⁺ and Na⁺ at a common site.

Functionally cloned pdrM from Streptococcus pneumoniae encodes a Na(+) coupled multidrug efflux pump

Multidrug efflux pumps play an important role as a self-defense system in bacteria. Bacterial multidrug efflux pumps are classified into five families based on structure and coupling energy: resistance−nodulation−cell division (RND), small multidrug resistance (SMR), major facilitator (MF), ATP binding cassette (ABC), and multidrug and toxic compounds extrusion (MATE). We cloned a gene encoding a MATE-type multidrug efflux pump from Streptococcus pneumoniae R6, and designated it pdrM. PdrM showed sequence similarity with NorM from Vibrio parahaemolyticus, YdhE from Escherichia coli, and other bacterial MATE-type multidrug efflux pumps. Heterologous expression of PdrM let to elevated resistance to several antibacterial agents, norfloxacin, acriflavine, and 4′,6-diamidino-2-phenylindole (DAPI) in E. coli KAM32 cells. PdrM effluxes acriflavine and DAPI in a Na⁺- or Li⁺-dependent manner. Moreover, Na⁺ efflux via PdrM was observed when acriflavine was added to Na⁺-loaded cells expressing pdrM. Therefore, we conclude that PdrM is a Na⁺/drug antiporter in S. pneumoniae. In addition to pdrM, we found another two genes, spr1756 and spr1877,that met the criteria of MATE-type by searching the S. pneumoniae genome database. However, cloned spr1756 and spr1877 did not elevate the MIC of any of the investigated drugs. mRNA expression of spr1756, spr1877, and pdrM was detected in S. pneumoniae R6 under laboratory growth conditions. Therefore, spr1756 and spr1877 are supposed to play physiological roles in this growth condition, but they may be unrelated to drug resistance.

Reduced Na+ affinity increases turnover of Salmonella enterica serovar Typhimurium MelB

The melibiose permease of Salmonella enterica serovar Typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+). Bioinformatics and mutational analyses indicate that a conserved Gly117 (helix IV) is a component of the Na(+)-binding site. In this study, Gly117 was mutated to Ser, Asn, or Cys. All three mutations increase the maximum rate (V(max)) for melibiose transport in Escherichia coli DW2 and greatly decrease Na(+) affinity, indicating that intracellular release of Na(+) is facilitated. Rapid melibiose transport, particularly by the G117N mutant, triggers osmotic lysis in the lag phase of growth. The findings support the previous conclusion that Gly117 plays an important role in cation binding and translocation. Furthermore, a spontaneous second-site mutation (P148L between loop(4-5) and helix V) in the G117C mutant prevents cell lysis. This mutation significantly decreases V(max) with little effect on cosubstrate binding in G117C, G117S, and G117N mutants. Thus, the P148L mutation specifically inhibits transport velocity and thereby blocks the lethal effect of elevated melibiose transport in the Gly117 mutants.

Role of Gly117 in the cation/melibiose symport of MelB of Salmonella typhimurium

The melibiose permease of Salmonella typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+), and bioinformatics analysis indicates that a conserved Gly117 (helix IV) is part of the Na(+)-binding site. We mutated Gly117 to Ala, Pro, Trp, or Arg; the effects on melibiose transport and binding of cosubstrates depended on the physical-chemical properties of the side chain. Compared with WT MelB(St), the Gly117 → Ala mutant exhibited little difference in either cosubstrate binding or stimulation of melibiose transport by Na(+) or Li(+), but all other mutations reduced melibiose active transport and efflux, and decreased the apparent affinity for Na(+). The bulky Trp at position 117 caused the greatest inhibition of melibiose binding, and Gly117 → Arg yielded less than a 4-fold decrease in the apparent affinity for melibiose at saturating Na(+) or Li(+) concentration. Remarkably, the mutant Gly117 → Arg catalyzed melibiose exchange in the presence of Na(+) or Li(+), but did not catalyze melibiose translocation involving net flux of the coupling cation, indicating that sugar is released prior to release of the coupling cation. Taken together, the findings are consistent with the notion that Gly117 plays an important role in cation binding and translocation.

Mechanism of melibiose/cation symport of the melibiose permease of Salmonella typhimurium

The MelB permease of Salmonella typhimurium (MelB-ST) catalyzes the coupled symport of melibiose and Na(+), Li(+), or H(+). In right-side-out membrane vesicles, melibiose efflux is inhibited by an inwardly directed gradient of Na(+) or Li(+) and stimulated by equimolar concentrations of internal and external Na(+) or Li(+). Melibiose exchange is faster than efflux in the presence of H(+) or Na(+) and stimulated by an inwardly directed Na(+) gradient. Thus, sugar is released from MelB-ST externally prior to the release of cation in agreement with current models proposed for MelB of Escherichia coli (MelB-EC) and LacY. Although Li(+) stimulates efflux, and an outwardly directed Li(+) gradient increases exchange, it is striking that internal and external Li(+) with no gradient inhibits exchange. Furthermore, Trp → dansyl FRET measurements with a fluorescent sugar (2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside) demonstrate that MelB-ST, in the presence of Na(+) or Li(+), exhibits (app)K(d) values of ∼1 mM for melibiose. Na(+) and Li(+) compete for a common binding pocket with activation constants for FRET of ∼1 mM, whereas Rb(+) or Cs(+) exhibits little or no effect. Taken together, the findings indicate that MelB-ST utilizes H(+) in addition to Na(+) and Li(+). FRET studies also show symmetrical emission maximum at ∼500 nm with MelB-ST in the presence of 2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside and Na(+), Li(+), or H(+), which implies a relatively homogeneous distribution of conformers of MelB-ST ternary complexes in the membrane.

The Inner Interhelix Loop 4–5 of the Melibiose Permease from Escherichia coli Takes Part in Conformational Changes after Sugar Binding

Cytoplasmic loop 4-5 of the melibiose permease from Escherichia coli is essential for the process of Na+-sugar translocation (Abdel-Dayem, M., Basquin, C., Pourcher, T., Cordat, E., and Leblanc, G. (2003) J. Biol. Chem. 278, 1518-1524). In the present report, we analyze functional consequences of mutating each of the three acidic amino acids in this loop into cysteines. Among the mutants, only the E142C substitution impairs selectively Na+-sugar translocation. Because R141C has a similar defect, we investigated these two mutants in more detail. Liposomes containing purified mutated melibiose permease were adsorbed onto a solid supported lipid membrane, and transient electrical currents resulting from different substrate concentration jumps were recorded. The currents evoked by a melibiose concentration jump in the presence of Na+, previously assigned to an electrogenic conformational transition (Meyer-Lipp, K., Ganea, C., Pourcher, T., Leblanc, G., and Fendler, K. (2004) Biochemistry 43, 12606-12613), were much smaller for the two mutants than the corresponding signals in cysteineless MelB. Furthermore, in R141C the stimulating effect of melibiose on Na+ affinity was lost. Finally, whereas tryptophan fluorescence spectroscopy revealed impaired conformational changes upon melibiose binding in the mutants, fluorescence resonance energy transfer measurements indicated that the mutants still show cooperative modification of their sugar binding sites by Na+. These data suggest that: 1) loop 4-5 contributes to the coordinated interactions between the ion and sugar binding sites; 2) it participates in an electrogenic conformational transition after melibiose binding that is essential for the subsequent obligatory coupled translocation of substrates. A two-step mechanism for substrate translocation in the melibiose permease is suggested.

The gusBC genes of Escherichia coli encode a glucuronide transport system

Two genes, gusB and gusC, from a natural fecal isolate of Escherichia coli are shown to encode proteins responsible for transport of beta-glucuronides with synthetic [(14)C]phenyl-1-thio-beta-d-glucuronide as the substrate. These genes are located in the gus operon downstream of the gusA gene on the E. coli genome, and their expression is induced by a variety of beta-d-glucuronides. Measurements of transport in right-side-out subcellular vesicles show the system has the characteristics of secondary active transport energized by the respiration-generated proton motive force. When the genes were cloned together downstream of the tac operator-promoter in the plasmid pTTQ18 expression vector, transport activity was increased considerably with isopropylthiogalactopyranoside as the inducer. Amplified expression of the GusB and GusC proteins enabled visualization and identification by N-terminal sequencing of both proteins, which migrated at ca. 32 kDa and 44 kDa, respectively. Separate expression of the GusB protein showed that it is essential for glucuronide transport and is located in the inner membrane, while the GusC protein does not catalyze transport but assists in an as yet unknown manner and is located in the outer membrane. The output of glucuronides as waste by mammals and uptake for nutrition by gut bacteria or reabsorption by the mammalian host is discussed.

Sugar binding induced charge translocation in the melibiose permease from Escherichia coli

Electrogenic events associated with the activity of the melibiose permease (MelB), a transporter from Escherichia coli, were investigated. Proteoliposomes containing purified MelB were adsorbed to a solid supported lipid membrane, activated by a substrate concentration jump, and transient currents were measured. When the transporter was preincubated with Na(+) at saturating concentrations, a charge translocation in the protein upon melibiose binding could still be observed. This result demonstrates that binding of the uncharged substrate melibiose triggers a charge displacement in the protein. Further analysis showed that the charge displacement is neither related to extra Na(+) binding to the transporter, nor to the displacement of already bound Na(+) within the transporter. The electrogenic melibiose binding process is explained by a conformational change with concomitant displacement of charged amino acid side chains and/or a reorientation of helix dipoles. A kinetic model is suggested, in which Na(+) and melibiose binding are distinct electrogenic processes associated with approximately the same charge displacement. These binding reactions are fast in the presence of the respective cosubstrate (k > 50 s(-1)).

Evidence for intraprotein charge transfer during the transport activity of the melibiose permease from Escherichia coli

Electrogenic activity associated with the activity of the melibiose permease (MelB) of Escherichia coli was investigated by using proteoliposomes containing purified MelB adsorbed onto a solid-supported membrane. Transient currents were selectively recorded by applying concentration jumps of Na+ ions (or Li+) and/or of different sugar substrates of MelB (melibiose, thio-methyl galactoside, raffinose) using a fast-flow solution exchange system. Characteristically, the transient current response was fast, including a single decay exponential component (tau approximately 15 ms) on applying a Na+ (or Li+) concentration jump in the absence of sugar. On imposing a Na+ (or Li+) jump on proteoliposomes preincubated with the sugar, a sugar jump in a preparation preincubated with the cation, or a simultaneous jump of the cation and sugar substrates, the electrical transients were biphasic and comprised both the fast and an additional slow (tau approximately 350 ms) decay components. Finally, selective inactivation of the cosubstrate translocation step by acylation of MelB cysteins with N-ethyl maleimide suppressed the slow response components and had no effect on the fast transient one. We suggest that the fast transient response reflects charge transfer within MelB during cosubstrate binding while the slow component is associated with charge transfer across the proteoliposome membrane. From the time course of the transient currents, we estimate a rate constant for Na+ binding in the absence and presence of melibiose of k > 50 s(-1) and one for melibiose binding in the absence of Na+ of k approximately 10 s(-1).

Expression of sodium/proton antiporter NhaA at various pH values in Escherichia coli

It was reported that NhaA, one of sodium/proton antiporters in Escherichia coli, was expressed at alkaline pH [J. Biol. Chem. 266 (1991) 21753]. In disagreement with their results, expression of an nhaA-lacZ fusion gene was found to be very low in an E. coli strain derived from MC4100 within the wide pH range from 5 to 9. When nhaB was deleted, the fusion gene was expressed at pH values below 8, while the expression was observed at alkaline pH after chaA was deleted. The internal level of sodium ions was increased by deletion of nhaA in strains deficient in nhaB and chaA at low and high pH values, respectively. These results suggested that nhaA is induced only when a low level of internal sodium ions is not kept by NhaB and ChaA. Strains used in the previous study may have low active ChaA.

The proximity between helix I and helix XI in the melibiose carrier of Escherichia coli as determined by cross-linking

The melibiose carrier of Escherichia coli is a transmembrane protein that comprises 12 transmembrane helices connected by periplasmic and cytoplasmic loops, with both the N- and C-termini located on the cytoplasmic side. Our previous studies of second-site revertants suggested proximity between several helices, including helices XI and I. In this study, we constructed six double cysteine mutants, each having one cysteine in helix I and the other in helix XI: three mutants, K18C/S380C, D19C/S380C, and F20C/S380C, have their cysteine pairs near the cytoplasmic side of the carrier, and the other three, T34C/G395C, D35C/G395C, and V36C/G395C, have their cysteine pairs near the periplasmic side. In the absence of substrate, disulfide formations catalyzed by iodine and copper-(1,10-phenanthroline)(3) indicate that helix I and helix XI are in immediate proximity to each other on the periplasmic side but not on the cytoplasmic side, as shown by protease cleavage analyses. We infer that the two helices are tilted with respect to each other, with the periplasmic sides in close proximity.

The effect of modifications of the charged residues in the transmembrane helices on the transport activity of the melibiose carrier of Escherichia coli

The melibiose transport carrier of Escherichia coli (coded by melB gene) is a cotransport system which couples the transport of a-galactosides to protons, sodium, or lithium ions. The charged amino acid residues in membrane-spanning helices are of considerable interest because many of them have important function in substrate recognition. In most cases changing these charged residue to an uncharged residue (cysteine) results in total loss of activity. In this communication we describe experiments in which the cysteine substitution for a charged residue was chemically changed by sulfhydryl reagents (MTSEA and MTSET to restore a positive charge and MTSES a negative charge) or by iodoacetic acid or through oxidation by hydrogen peroxide so as to regain the original negative charge. In two cases (D55C and D124C) the reconstructed negative charges via the oxidation of the thiol to the sulfinic and/or sulfonic acid resulted in partial recovery of transport: D55C up to 27% of the normal and D124C up to 4% of the normal in melibiose accumulation; D55C up to 36% of the normal and D124 up to 4.5% of the normal in downhill transport. Sulfhydryl reagents and iodoacetic acid failed to recover transport in all cases. We infer that the configurations of the charges as well as the structure of the side chains that carry them are critical in the maintenance of the transport.

Sodium-substrate cotransport in bacteria

A variety of sodium-substrate cotransport systems are known in bacteria. Sodium enters the cell down an electrochemical concentration gradient. There is obligatory coupling between the entry of the ion and the entry of substrate with a stoichiometry (in the cases studied) of 1:1. Thus, the downhill movement of sodium ion into the cell leads to the accumulation of substrate within the cell. The melibiose carrier of Escherichia coli is perhaps the most carefully studied of the sodium cotransport systems in bacteria. This carrier is of special interest because it can also use protons or lithium ions for cotransport. Other sodium cotransport carriers that have been studied recently are for proline, glutamate, serine-threonine, citrate and branched chain amino acids.

Physiological evidence for an interaction between helix XI and helices I, II, and V in the melibiose carrier of Escherichia coli

In a previous study 23 residues in helix XI of the cysteine-less melibiose carrier were changed individually to cysteine. Several of these cysteine mutants (K377C, A383C, F385C, L391C, G395C) had low transport activity and they were white on melibiose MacConkey fermentation plates. After several days of incubation of these white clones on melibiose MacConkey plates a rare red mutant appeared. The plasmid DNA was then isolated and sequenced. The two second site revertants from K377C were I22S and D59A. This change of aspartic acid to a neutral residue suggests that physiologically there is an interaction between K377 and D59 (possibly a salt bridge). The revertants from A383C were in positions 20 (F20L) and 22 (I22S and I22N). Revertants of F385C were intrahelical changes (I387M and A388G) and a change in C-terminal loop (R441C). Revertants of L391C were in helix I (I22N, I22T and D19E) and helix V (A152S). Revertants of G395C were in helix I (D19E and I22N). We suggest that there is an interaction between helix XI and helices I, II, and V and proximity between these helices.

Structural analysis of a plant sucrose carrier using monoclonal antibodies and bacteriophage lambda surface display

Monoclonal antibodies were raised and selected against recombinant Plantago major PmSUC2 sucrose carrier protein. Epitopes of two monoclonal antibodies (PS2-1A2 and PS2-4D4) were mapped using N-terminally truncated PmSUC2 proteins and a lambda library displaying random PmSUC2 peptides. PS2-1A2 recognizes an octapeptide close to the N-terminus of PmSUC2, PS2-4D4 binds to a decapeptide at the very C-terminus. Analyses of antibody binding to yeast protoplasts with functionally active, tagged PmSUC2 protein revealed that both epitopes are located in cytoplasmic domains of PmSUC2. These results support a model for plant sucrose transporters containing 12 transmembrane helices with the N-terminus and the C-terminus on the cytoplasmic side of the plasma membrane.

Conversion of temperature-sensitive to -resistant gene expression due to mutations in the promoter region of the melibiose operon in Escherichia coli

The melibiose utilization system of Escherichia coli W3133, a derivative of K12, is nonfunctional between 37 and 42 degreesC. The reason for this temperature sensitivity was thought to be that the melibiose transporter (MelB) of W3133 cells was temperature-sensitive. A mutant W3133-2 has been isolated as a temperature-resistant strain that can utilize melibiose between 37 and 42 degreesC. However, we found that the melibiose transporter of the W3133-2 was still temperature-sensitive. Half-life activities of the melibiose transporter at 37 degreesC (or 40 degreesC) in both E. coli W3133 and W3133-2 were exactly the same. Furthermore, we found that the nucleotide sequence of coding region of the melB structural gene (the second gene of the melibiose operon) of W3133-2 was exactly the same as that of W3133. Activity of alpha-galactosidase (product of the first gene, melA, of the melibiose operon) of W3133 cells grown at 40 degreesC was very low, although that of W3133-2 cells grown at 40 degreesC was high. These observations suggested that expression of the melibiose operon in W3133 is also temperature-sensitive. In fact, we found that the expression in W3133 cells was temperature-sensitive, while that in W3133-2 cells was temperature-resistant, by analyzing mRNA levels using the Northern blot method. Furthermore, we identified mutations in the promoter region of the melibiose operon of W3133-2 that resulted in the elongation of an 18 nucleotide inverted repeat sequence to a 28-nucleotide repeat sequence present immediately upstream of the -35 region. This may stabilize a possible stem structure due to the inverted repeat at 37-42 degreesC.

Mutants of Citrobacter freundii that transport and utilize melibiose

We have isolated mutants of Citrobacter freundii that can grow on melibiose. Inducible alpha-galactosidase activity and melibiose transport activity were detected in the mutant cells but not in the wild-type cells. We detected a DNA region which hybridized with melB (the gene for the melibiose transporter) DNA of Escherichia coli in the chromosomal DNA of wild-type C. freundii. Protons, but not sodium ions, were found to be the coupling cations for melibiose (and methyl-beta-D-thiogalactoside) transport in the mutant cells.

The construction of a cysteine-less melibiose carrier from E. coli

The melibiose carrier of E. coli is a cation-sugar cotransport system. This membrane protein contains four cysteine residues and the transport function is inhibited by sulfhydryl reagents. In order to investigate the importance of the cysteines, we have constructed a set of four melibiose transporters each of which has one cysteine replaced with serine or valine. The sensitivity of this set of carriers to N-ethylmaleimide was tested and Cys364 was identified as the target of the reagent. In addition, we constructed a melibiose transporter in which all 4 cysteines were replaced with either serine (Cys110, Cys310, and Cys364) or valine (Cys235) and we found that, as expected, the resulting cysteine-less transporter was resistant to the action of N-ethylmaleimide. The cysteine-less melibiose carrier had no significant decrease in ability to accumulate melibiose with cotransported sodium ions or protons. Thus, none of the 4 cysteines are necessary for the function of the melibiose carrier.

Sequence of a melibiose transporter gene of Enterobacter cloacae

We cloned a fragment of the chromosomal DNA of Enterobacter cloacae, which enabled a melibiose-negative Escherichia coli mutant lacking melB to grow on melibiose as the sole source of carbon. Transformed cells harboring the hybrid plasmid carrying the cloned DNA showed melibiose transport activity. The nucleotide sequence of the DNA region was determined. One complete open reading frame (ORF) and a part of another ORF were found in the region, and the amino acid sequences were deduced. The complete ORF was found to encode a melibiose transporter which consisted of 425 amino acid residues. Hydropathy analysis revealed that there are about 12 hydrophobic domains in this transporter. The incomplete ORF which exists in the upstream region of the transporter gene seemed to encode an alpha-galactosidase.

A melibiose transporter and an operon containing its gene in Enterobacter cloacae

We detected inducible melibiose transport activity in cells of Enterobacter cloacae IID977. H+, but not Na+, was found to be the coupling cation for this transporter. We cloned and sequenced the gene encoding the melibiose transporter. A homology search of a protein sequence database revealed that this melibiose transporter has high sequence similarity with the lactose transporter (LacY) and the raffinose transporter (RafB) and has some similarity with the melibiose transporter (MelB) of Escherichia coli.

Characteristics of the melibiose transporter and its primary structure in Enterobacter aerogenes

Cells of Enterobacter aerogenes can grow on melibiose as a sole source of carbon. This suggests the presence of melibiose operon in this organism. We found that E. aerogenes cells possess both alpha-galactosidase activity and melibiose transport activity, which were induced by melibiose. Neither Na+ nor Li+ stimulated the melibiose transport. However, transport of methyl-beta-thiogalactoside (TMG) was stimulated by Li+ but not by Na+. These findings suggest that the major coupling cation for the melibiose transporter in E. aerogenes is H+. In fact, we observed H+ entry into cells caused by an influx of melibiose and some of its analogs. We cloned the melB gene which encodes the melibiose transporter, and sequenced it. Deduced amino acid sequence of the transporter revealed that the melibiose transporter consists of 471 amino acid residues and the molecular weight was calculated to be 52214 Da. The sequence showed high homology with the sequences of the melibiose transporters of Escherichia coli, Salmonella typhimurium and Klebsiella pneumoniae. Higher homology was found with the melibiose transporter of K. pneumoniae than with that of E. coli and S. typhimurium.

Inhibition of melibiose transporter by amiloride in Escherichia coli

Amiloride inhibited the active transport of melibiose via the melibiose transporter in Escherichia coli. Since amiloride is known to inhibit the Na+/H+ antiporter which is essential for the establishment of an electrochemical potential of Na+ that is the driving force for melibiose transport, we tested the effect of amiloride on the activity of the melibiose transporter itself. Amiloride inhibited the melibiose counterflow. Also, inhibition of Na+ uptake induced by melibiose influx and some inhibition of H+ uptake induced by melibiose influx were observed. These results indicate that amiloride directly inhibits the melibiose transporter, perhaps by competing with Na+. It seems that the Na+ binding site and the H+ binding site in the melibiose transporter are somehow different from each other judging from the difference in the inhibition pattern of amiloride.

Melibiose permease of Escherichia coli: structural organization of cosubstrate binding sites as deduced from tryptophan fluorescence analyses

Binding of the coupling ion (Na+ or Li+) and sugars to the purified melibiose permease of Escherichia coli, reconstituted in proteoliposomes, produces selective and cooperative changes of the transporter tryptophan fluorescence. To assess the individual contribution of N- or C-terminal domains of the permease to these substrate-induced fluorescence variations, we replaced the two tryptophans located in its C-terminal half (W299 and W342) by a phenylalanine and compared the signal change in mutants and wild-type permease. None of the mutations significantly impairs transport activity. Persistence of the ion-induced signal quenching in a permease carrying only the six other tryptophans of the N-terminal domain is consistent with a previous suggestion that this domain accommodates the ion-binding site. On the other hand, the sugar-induced fluorescence increase varies from mutant to mutant in a sugar-specific fashion. While alpha-galactosides increase essentially the fluorescence of W299 and W342, beta-galactosides enhance the signal of W299 and of one (or more) of the N-terminal tryptophans but quench that of W342. Moreover, addition of sugars producers a 10 nm blue shift of both W299 and W342 emission spectra, suggesting reduced accessibility of these residues to solvent following substrate binding. These data suggest that W299 and W342 are at or close to the sugar binding site and that this latter is lined by the C-terminal helices IX and X. Moreover, as sugars with the beta-configuration also enhance the fluorescence of the N-terminal tryptophans, it is suggested that one (or more) helix of the N-terminal half may be also at or near the sugar binding site. This implies close proximity and/or tight functional linkage between some N-terminal helices and helices IX and X of the C-terminal domain of the transporter.

Alteration of Na(+)-coupled transport in site-directed mutants of the melibiose carrier of Escherichia coli

Asn-58 of the Escherichia coli melibiose carrier was replaced by Ala, Leu, Ser, and Gln. Trp-54 was replaced by Leu and a double mutant Leu-54/Ala-58 was constructed using site-directed mutagenesis. Cation/sugar cotransport and sugar-induced cation uptake were studied for each mutant. The change of Asn-58 to Ala results in a nearly complete loss of Na(+)-stimulated galactoside transport as well as sugar-stimulated Na+ uptake. Substitutions of Leu, Gln, and Ser for Asn-58 were also defective in Na(+)-stimulated sugar transport. The Trp-54 to Leu mutant shows moderate sugar accumulation with cation selectivity similar to wild-type. The double mutant Leu-54/Ala-58 shows elevated H(+)-melibiose cotransport as well as reduced Na(+)-stimulated melibiose cotransport. These results suggest that Asn-58 is important for Na+ recognition.

Membrane topology of the melibiose permease of Escherichia coli studied by melB-phoA fusion analysis

In order to study the secondary structure of the melibiose permease of Escherichia coli, 57 melB-phoA gene fusions were constructed and assayed for alkaline phosphatase activity. In general agreement with a previously suggested secondary structure model of melibiose permease [Botfield, M. C., Naguchi, K., Tsuchiya, T., & Wilson, T.H. (1992) J. Biol. Chem. 267, 1818], clusters of fusions exhibiting low and high phosphatase activity fusions alternate along the primary sequence. Fusions with high activity generally cluster at residues predicted to be in the periplasmic half of transmembrane domains or in periplasmic loops, while fusions with low activity cluster at residues predicted to be in the cytoplasmic half of transmembrane domains or in cytoplasmic loops. Taken together, the findings strongly support the contention that melibiose permease contains 12 transmembrane domains that traverse the membrane in zigzag fashion connected by hydrophilic loops that are exposed alternatively on the periplasmic or cytoplasmic surfaces of the membrane with the N and C termini on the cytoplasmic face of the membrane. Moreover, on the basis of the finding that the cytoplasmic half of an out-going segment is sufficient for alkaline phosphatase export to the periplasm while the periplasmic half of an in-going segment prevents it [Calamia, T., & Manoil, C. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 4937], the activity profile of the melibiose permease-alkaline phosphatase fusions is consistent with the predicted topology of seven of 12 transmembrane segments. However, five transmembrane domains require adjustment, and as a consequence, the size of the central cytoplasmic loop is reduced and a significant number of charged residues are shifted from a hydrophilic to a hydrophobic domain in this region of the transporter.

Properties of a Na+/galactose (glucose) symport system in Vibrio parahaemolyticus

We have investigated galactose transport in a mutant strain of Vibrio parahaemolyticus that lacks a glucose-PTS (phosphoenolpyruvate:carbohydrate phosphotransferase system) and a trehalose-PTS. Cells of the V. parahaemolyticus actively transported D-galactose and Na+ greatly stimulated the transport. Maximum stimulation of D-galactose transport activity was observed at 10mM NaCl, and Na+ could be replaced with Li+. Addition of galactose to the cell suspension under anaerobic conditions elicited Na+ uptake. Therefore, we conclude that this organism accomplishes galactose transport by a Na+/solute symport mechanism. Judging from inhibition results, D-galactose, D-glucose and to a lesser extent alpha-D-fucose are substrates of this transport system. The Na+/galactose symport system exhibited a high affinity for D-galactose (Km: 40 microM) and showed a relatively lower affinity for D-glucose (Km: 420 microM), but the maximum velocities for galactose and glucose transport were almost same (about 52 nmol/min per mg protein). The Na+/D-galactose symport system was induced by either D-galactose or alpha-D-fucose, and repressed by D-glucose.

Characterisation of glucose transport in Helicobacter pylori

The nature of the glucose transport system in the bacterium Helicobacter pylori was investigated employing radioactive tracer analysis. Fast D-glucose uptake was demonstrated by using two methods of measuring glucose transport. The transport of 2-deoxy-D-glucose was inhibited competitively by D-glucose; and the efflux of 2-deoxy-D-glucose from cells also was affected by the presence of D-glucose. The transport of 2-deoxy-D-glucose was saturable with a Km of 4.8 mM and Vmax of 146.6 pmol (microliter cell water)-1 at 20 degrees C. The transport was temperature-dependent with energies of activation of 6.8 and 51.0 kJ mol-1 for 0.2 and 20 mM 2-deoxy-D-glucose, respectively. The temperature dependence and saturable nature of transport suggested the presence of one or more glucose carriers. The substrate specificity of the transport system was studied by measuring the effects of mono- and disaccharides on the rates of transport of the glucose analogue. The most significant inhibitory effects were obtained with D-galactose and L-arabinose. Lack of transport inhibition by L-glucose established the stereospecificity of the transporters for the D-isomer of glucose. Higher rates of 2-deoxy-D-glucose transport were measured in the presence of sodium ions than for other monovalent cations, and the presence of amiloride inhibited transport of the monosaccharide. No inhibition was observed with cytochalasin B, phloretin or phloridzin. The results suggested the existence of specific D-glucose transporters and that the glucose transport system of H. pylori is significantly different from other known bacterial transporters.

Melibiose permease of Escherichia coli: substrate-induced conformational changes monitored by tryptophan fluorescence spectroscopy

Tryptophan fluorescence spectroscopy has been used to investigate the effects of sugars and coupling cations (H+, Na+, or Li+) on the conformational properties of purified melibiose permease after reconstitution in liposomes. Melibiose permease emission fluorescence is selectively enhanced by sugars, which serve as substrates for the symport reaction, alpha-galactosides producing larger variations (13-17%) than beta-galactosides (7%). Moreover, the sugar-dependent fluorescence increase is specifically potentiated by NaCl and LiCl (5-7 times), which are well-established activators of sugar binding and transport by the permease. The potentiation effect is greater in the presence of LiCl than NaCl. On their own, sodium and lithium ions produce quenching of the fluorescence signal (2%). Evidence suggesting that sugars and cations compete for their respective binding sites is also given. Both the sugar-induced fluorescence variation and the NaCl(or LiCl)-dependent potentiation effect exhibit saturation kinetics. In each ionic condition, the half-maximal fluorescence change is found at a sugar concentration corresponding to the sugar-binding constant. Also, half-maximal potentiation of the fluorescence change by sodium or lithium occurs at a concentration comparable to the activation constant of sugar binding by each ion. The sugar- and ion-dependent fluorescence variations still take place after selective inactivation of the permease substrate translocation capacity by N-ethylmaleimide. Taken together, the data suggest that the changes in permease fluorescence reflect conformational changes occurring upon the formation of ternary sugar/cation/permease complexes.

Characterization of a glucose transport system in Vibrio parahaemolyticus

Cells of a glucose-PTS (phosphoenolpyruvate:carbohydrate phosphotransferase system)-negative mutant of Vibrio parahaemolyticus transport D-glucose in the presence of Na+. Maximum stimulation of D-glucose transport was observed at 40 mM NaCl, and Na+ could be replaced partially with Li+. Addition of D-glucose to the cell suspension under anaerobic conditions elicited Na+ uptake. Thus, we conclude that glucose is transported by a Na+/glucose symport mechanism. Calculated Vmax and Km values for the Na(+)-dependent D-glucose transport were 15 nmol/min/mg of protein and 0.57 mM, respectively, when NaCl was added at 40 mM. Na+ lowered the Km value without affecting the Vmax value. D-Glucose was the best substrate for this transport system, followed by galactose, alpha-D-fucose, and methyl-alpha-glucoside, judging from the inhibition pattern of the glucose transport. D-Glucose itself partly repressed the transport system when cells were grown in its presence.

Properties of recombinant cells capable of growing on serine without NhaB Na+/H+ antiporter in Escherichia coli

Escherichia coli HIT-1 has a mutation in the Na+/H+ antiporter gene, nhaB (P. Thelen, T. Tsuchiya, and E. B. Goldberg, J. Bacteriol. 173:6553-6557, 1991). This strain is not able to utilize serine as a carbon source (T. Ishikawa, H. Hama, M. Tsuda, and T. Tsuchiya, J. Biol. Chem. 262:7443-7446, 1987), because an active NhaB is required to maintain the electrochemical potential of Na+, which drives serine transport via the Na+/serine carrier, the major transport system for serine. We isolated recombinant cells from a cross between strains HIT-1 and Hfr, and these cells were able to grow on serine even though the NhaB Na+/H+ antiporter of the recombinant cells was still defective. We found that the activity of the H+/serine cotransport system, one of the minor serine transport systems in E. coli, was elevated in the recombinant cells. H+/serine cotransport activity was induced by leucine in the recombinant cells more strongly than in strain HIT-1. A kinetic analysis showed that the Vmax, but not the Km, of the transport system was much higher in the recombinant cells than in strain HIT-1 cells.

Melibiose transport system in Lactobacillus plantarum

Lactobacillus plantarum ATCC 8014 grew on melibiose at 30 C, but not at 37 C, although it grew on galactose or lactose at either temperature. ATCC 8014 grown on lactose at 30 or 37 C accumulated melibiose slowly, suggesting that melibiose may partly be transported by a lactose transport system. A lactose-negative mutant, NTG 21, derived from ATCC 8014 was isolated. The mutant was totally deficient in lactose transport, but retained normal melibiose transport activity. In NTG 21, the melibiose transport activity was induced by melibiose at 30 C, but not at 37 C. The transport activity itself was found to be stable for at least 3 hr at 37 C, suggesting that the induction process in the cytoplasm rather than the inducer entrance is temperature-sensitive in the organism. The organism also failed to form alpha-galactosidase at 37 C when grown on melibiose. The enzyme synthesis, however, was induced by galactose in NTG 21 (and also by lactose in ATCC 8014) even at 37 C, indicating that the induction of the enzyme is essentially not temperature-sensitive. In NTG 21, melibiose transport system and alpha-galactosidase were induced by galactose, melibiose and o-nitrophenyl-alpha-D-galactopyranoside when the strain was grown at 30 C. Raffinose induced melibiose transport system only a little, while it was a good inducer for alpha-galactosidase. Inhibition studies revealed that galactose may be a weak substrate of the melibiose transport system; no inhibition was demonstrated with lactose and raffinose.

Assessment of uncoupling by amiloride analogs

The amiloride analogs N5-methyl-N5-isobutylamiloride, N5-ethyl-N5-isopropylamiloride, and N5,N5-hexamethyleneamiloride are frequently used to investigate NaH exchange on the premise that they are highly specific inhibitors of the NaH-antiporters. We assessed the relative protonophoric activity of these compounds in reconstituted and native membrane vesicles, using acridine orange fluorescence to measure intravesicular pH. All the compounds tested were found to be potent protonophores at concentrations which are normally used to demonstrate inhibition of NaH exchange. Uncoupling was dependent on both the pH of the assay system and the total amount of lipid present. At the pH optima, which lay in a range from 7.5 to 8.5, these amiloride analogs were more potent uncouplers than the classical protonophore carbonyl cyanide m-chlorophenylhydrazone. Therefore, extreme care must be taken in the interpretation of results obtained using these or similar derivatives of amiloride.

Cloning and sequencing of the melB gene encoding the melibiose permease of Salmonella typhimurium LT2

The nucleotide sequence of the melB gene coding for the Na+ (Li+)/melibiose symporter of Salmonella typhimurium LT2 was determined, and its amino acid sequence was deduced. It consists of 1428 bp, corresponding to a protein of 476 amino acid residues (calculated molecular weight 52,800). The amino acid sequence is homologous to that of the melibiose permease of Escherichia coli K12, with 85% identical residues. All, except one, of the amino acid residues that have been reported to be important for cation or substrate recognition in the melibiose permease of E. coli are conserved in the melibiose permease of S. typhimurium. In addition, part of the sequence resembles the lactose permease of Streptococcus thermophilus, the animal glucose transporter (GLUT1), the plasmid-coded raffinose permease (RafB), and the NADH-ubiquinone oxidoreductase chain 4 (Nuo4) of Aspergillus amstelodami.

Melibiose permease of Escherichia coli: mutation of histidine-94 alters expression and stability rather than catalytic activity

Previous studies utilizing site-directed mutagenesis [Pourcher et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 468-472] indicate that out of seven histidinyl residues in the melibiose (mel) permease of Escherichia coli, only His94 is important. The role of His94 has now been investigated by replacing the residue with Asn, Gln, or Arg. Cells expressing mel permease with Asn94 or Gln94 retain 30% or 20% of wild-type activity, respectively, and surprisingly, immunological assays demonstrate that diminished transport activity is due to a proportional reduction in the amount of permease in the membrane. Moreover, kinetic analyses of transport and ligand binding studies with right-side-out membrane vesicles indicate that both substrate recognition and turnover (kcat) are comparable in the mutant permeases and the wild-type. Mel permease with Arg in place of His94 also binds ligand and catalyzes sugar accumulation, but only when the cells are grown at 30 degrees C, and evidence is presented that Arg94 permease is inactivated at 37 degrees C. Finally, labeling studies demonstrate that expression and/or insertion of the permease, but not degradation, is strongly dependent on the amino acid present at position 94 and temperature. The findings indicate that an imidazole group at position 94 is required for proper insertion and stability of mel permease, but not for transport activity per se. Since replacement of the other six histidinyl residues in mel permease with Arg has little or no effect on transport activity, it is concluded that histidinyl residues do not play a direct role in the mechanism of this secondary transport protein.

Characterization and mapping of a major Na+/H+ antiporter gene of Escherichia coli

Using in vivo assays, we show that the Na+/H+ antiporter activity of the Escherichia coli mutant HIT-1 is reduced dramatically compared with activity in wild-type cells. An isogenic nhaA (formerly antA) deletion strain, however, is not significantly different from wild type in this respect. We call the locus affecting Na+/H+ antiporter activity of the HIT-1 mutant nhaB. The nhaB activity exhibits no pH dependence in the range between 7.0 and 8.5, whereas that of the nhaA gene increases considerably at pH levels above 8.0. Mutants with defects in nhaB grow normally on agar media containing 0.5 M NaCl, but nhaA mutants are sensitive to 0.5 M NaCl. We have mapped the nhaB mutation of HIT-1 to 25.6 min on the E. coli map. It is unlinked to the nhaA region, which is located at about 0.5 min. Since a cell with a mutation in nhaB alone is essentially Na+/H+ antiporter negative up to pH 8.0, we conclude that nhaB is required for the major Na+/H+ antiporter activity in the usual physiological pH range.